fix(sequential-thinking): Add input sanitization for numeric parameters #2812

ketema · 2025-10-03T09:52:18Z

Problem

Some MCP clients may serialize numeric parameters as strings in JSON payloads, causing validation errors with the current strict type checking.

Error Message:

{
  "error": "Invalid thoughtNumber: must be a number",
  "status": "failed"
}

Root Cause: The validateThoughtData() method performs strict type checking (typeof data.thoughtNumber !== 'number'), which fails when clients pass "1" (string) instead of 1 (number).

Solution

Add input sanitization to coerce valid numeric strings to numbers before validation, while maintaining type safety.

Changes

Added sanitizeNumericParam() private helper method
Sanitizes thoughtNumber, totalThoughts, revisesThought, branchFromThought
Coerces valid numeric strings (e.g., "1") to numbers (e.g., 1)
Preserves original validation logic after sanitization
Returns invalid values as-is for clear error messages

Testing

Handles number inputs: 1 → 1 (pass through)
Handles string inputs: "1" → 1 (coerce)
Handles invalid inputs: "abc" → "abc" (validation fails with clear error)

Impact

✅ Improves client compatibility
✅ Maintains type safety
✅ No breaking changes
✅ Clear error messages preserved

Example

Before:

// Client sends: { thoughtNumber: "1", ... }
// Result: Error: Invalid thoughtNumber: must be a number

After:

// Client sends: { thoughtNumber: "1", ... }
// Sanitized to: { thoughtNumber: 1, ... }
// Result: Success

Testing Evidence

Tested with Augment.AI MCP client:

✅ Accepts thoughtNumber: 1 (number)
✅ Accepts thoughtNumber: "1" (string, coerced to number)
✅ Rejects thoughtNumber: "abc" (invalid, clear error)
✅ Thought history maintained correctly
✅ No regressions observed

Additional Context

This fix addresses non-deterministic validation errors observed with MCP clients that use LLM-generated tool calls, where numeric parameters may be serialized as strings depending on the LLM's output format.

The sanitization approach:

Checks if value is already a number → pass through
Checks if value is a valid numeric string → parse and return number
Otherwise → return as-is, let validation fail with clear error

This maintains the original validation behavior while improving compatibility with clients that serialize numbers as strings.

WHY: Some MCP clients may pass numeric parameters as strings Strict type checking causes validation errors with 'Invalid thoughtNumber' EXPECTED: Sanitize valid numeric strings to numbers before validation Maintains type safety while improving client compatibility CHANGES: - Added sanitizeNumericParam() private helper method - Sanitizes thoughtNumber, totalThoughts, revisesThought, branchFromThought - Coerces valid numeric strings (e.g., '1') to numbers (e.g., 1) - Preserves original validation logic after sanitization - Returns invalid values as-is for clear error messages TESTING: - Handles number inputs: 1 → 1 (pass through) - Handles string inputs: '1' → 1 (coerce) - Handles invalid inputs: 'abc' → 'abc' (validation fails with clear error) Fixes: Non-deterministic validation errors with MCP clients that serialize numeric parameters as strings in JSON payloads

Copilot

Pull Request Overview

This PR improves client compatibility for the sequential-thinking MCP server by adding input sanitization for numeric parameters. The change addresses validation errors that occur when MCP clients serialize numeric values as strings in JSON payloads.

Key changes:

Added sanitizeNumericParam() helper method to coerce valid numeric strings to numbers
Updated validateThoughtData() to sanitize numeric parameters before validation
Maintained existing validation logic and error handling

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src/sequentialthinking/index.ts

WHY: Copilot review identified inconsistency between regex and validation Regex /^\d+$/ allowed '0' to match, but parsed > 0 rejected it EXPECTED: Regex explicitly excludes zero to match semantic intent Thought numbers are 1-indexed (1, 2, 3, ...), zero is invalid CHANGES: - Updated regex from /^\d+$/ to /^[1-9]\d*$/ - Added comments explaining thought numbers are 1-indexed - Regex now matches: 1, 2, 3, ... (excludes 0) - Maintains parsed > 0 check (now consistent with regex) COPILOT FEEDBACK ADDRESSED: 1. ✅ Regex inconsistency with zero - FIXED 2. ❌ Negative/decimal suggestion - Not applicable - Thought numbers are semantic indices (1st, 2nd, 3rd thought) - Decimals (1.5) and negatives (-1) don't make sense - Current regex already prevents negatives (no minus sign) TESTING: - Accepts: '1' → 1, '2' → 2, '999' → 999 - Rejects: '0' (fails regex), '-1' (fails regex), '1.5' (fails regex) Refs: PR modelcontextprotocol#2812 Copilot review

ketema · 2025-10-03T10:02:11Z

Thanks for the review, @Copilot!

Addressing your feedback:

✅ Comment 1: Regex inconsistency with zero - FIXED

You're absolutely right about the inconsistency. The regex /^\d+$/ allowed '0' to match, but the parsed > 0 condition rejected it.

Fix applied: Updated regex to /^[1-9]\d*$/ to explicitly exclude zero.

Rationale: Thought numbers are semantic indices representing "which thought in the sequence" (1st, 2nd, 3rd, etc.). Zero doesn't make sense in this context - thoughts are 1-indexed, not 0-indexed.

❌ Comment 2: Negative numbers and decimals - Not applicable

The suggestion to handle negative numbers and decimals doesn't apply here because:

Semantic context: These parameters represent thought indices (thoughtNumber, totalThoughts, etc.), not arbitrary numeric values
Negative numbers: Already prevented by the regex (no minus sign in pattern)
Decimals: Don't make sense for "which thought number" (can't have the 1.5th thought)

The function name sanitizeNumericParam is accurate - it sanitizes numeric parameters that should be positive integers starting from 1.

Updated behavior:

✅ Accepts: '1' → 1, '2' → 2, '999' → 999
❌ Rejects: '0' (fails regex), '-1' (fails regex), '1.5' (fails regex)

The fix maintains type safety while being explicit about the valid range for thought indices.

…sequential-thinking

WHY: - MCP SDK validates against tool schema BEFORE calling handler - Schema defined thoughtNumber/totalThoughts/etc as type: 'integer' - String values like '1' failed schema validation before reaching sanitizeNumericParam() - Input sanitization code was correct but never executed EXPECTED: - Schema now accepts both integers and strings for numeric parameters - MCP SDK allows strings to pass schema validation - sanitizeNumericParam() can coerce strings to numbers - Tool works with both thoughtNumber: 1 and thoughtNumber: '1' CHANGES: - Updated thoughtNumber schema: oneOf [integer, string with pattern] - Updated totalThoughts schema: oneOf [integer, string with pattern] - Updated revisesThought schema: oneOf [integer, string with pattern] - Updated branchFromThought schema: oneOf [integer, string with pattern] - Pattern: ^[1-9]\d*$ (matches positive integers, excludes zero) SCHEMA STRUCTURE: oneOf: [ { type: 'integer', minimum: 1 }, { type: 'string', pattern: '^[1-9]\d*$' } ] This allows MCP SDK to accept either: - Integer: 1, 2, 3, ... - String: '1', '2', '3', ... Both are then sanitized by sanitizeNumericParam() before validation. IMPACT: - Completes the input sanitization fix - Schema + sanitization + validation now work together - MCP clients can pass numeric parameters as strings or numbers - No more 'Invalid thoughtNumber: must be a number' errors for valid string numbers Refs: Schema validation happens before handler execution in MCP SDK

…ee-layer defense strategy WHY: - Complete fix requires understanding of BOTH schema update AND input sanitization - Schema oneOf prevents overly strict client-side validation from blocking LLM mistakes - Input sanitization provides server-side defense and type coercion - Together they ensure tool availability while maintaining correctness EXPECTED: - Clear documentation of root cause (client-side schema validation + missing sanitization) - Explanation of three-layer defense: permissive schema, sanitization, strict validation - Real-world scenarios showing why both fixes are required - Design philosophy: liberal in acceptance, strict in production Refs: MCP specification - clients SHOULD validate, servers MUST validate

WHY: - Sequential Thinking MCP server needs stable local build during development - Prevent NPM from auto-updating dependencies before PR merge - Ensure consistent build environment across development sessions EXPECTED: - NPM uses exact versions (save-exact=true) - Package-lock.json not modified automatically - Prefer offline cache to avoid registry fetches - No automatic dependency updates during local development Refs: Local development workflow for fix/input-sanitization-sequential-thinking branch

domdomegg

I think we don't plan to accept this change - this seems like a model bug that it is passing through the wrong shaped parameters. If we do want to correct this for poorly performing models, the place to do that would probably be the client anyways where it can be implemented once rather than server side.

ketema · 2025-10-10T17:45:14Z

I don’t have access to the clients and have noticed this issue in both Copilot, Augment, and Gemini. I felt this was simple fix for any client.

…

On Fri, Oct 10, 2025 at 04:27 adam jones ***@***.***> wrote: ***@***.**** requested changes on this pull request. I think we don't plan to accept this change - this seems like a model bug that it is passing through the wrong shaped parameters. If we do want to correct this for poorly performing models, the place to do that would probably be the client anyways where it can be implemented once rather than server side. — Reply to this email directly, view it on GitHub <#2812 (review)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAB7TGHCTA6H4L26MOW72HL3W6JZPAVCNFSM6AAAAACIGC5WT2VHI2DSMVQWIX3LMV43YUDVNRWFEZLROVSXG5CSMV3GSZLXHMZTGMRSG44DCMZSGY> . You are receiving this because you authored the thread.Message ID: ***@***.***>

olaservo · 2025-10-13T01:12:16Z

I don’t have access to the clients and have noticed this issue in both
Copilot, Augment, and Gemini. I felt this was simple fix for any client.

@ketema out of curiosity, what models are you using when this happens?

olaservo

Agree with Adam that not sure if forcing the values is the right thing to do here. I did have another question though about what models you're using when this happens.

olaservo · 2025-10-13T01:13:30Z

src/sequentialthinking/.npmrc

@@ -0,0 +1,19 @@
+# Local NPM configuration for Sequential Thinking MCP Server


Was this intentionally checked in?

olaservo · 2025-10-13T01:13:41Z

src/sequentialthinking/SCHEMA_FIX_EXPLANATION.md

@@ -0,0 +1,221 @@
+# Sequential Thinking MCP Server - Schema Fix Explanation


Was this intentionally checked in?

ketema · 2025-10-13T08:13:51Z

It has mostly happened with Anthropic Claude models and mostly with the Augment.ai agent, but as mentioned I have had it happen with copilot and Claude and once with Gemini in an early 1.5 flash model, although I wasn’t keeping strict count. It just happened enough that I decided to ask why then make the patch.

…

On Sun, Oct 12, 2025 at 21:15 Ola Hungerford ***@***.***> wrote: ***@***.**** commented on this pull request. Agree with Adam that not sure if forcing the values is the right thing to do here. I did have another question though about what models you're using when this happens. ------------------------------ In src/sequentialthinking/.npmrc <#2812 (comment)> : > @@ -0,0 +1,19 @@ +# Local NPM configuration for Sequential Thinking MCP Server Was this intentionally checked in? ------------------------------ In src/sequentialthinking/SCHEMA_FIX_EXPLANATION.md <#2812 (comment)> : > @@ -0,0 +1,221 @@ +# Sequential Thinking MCP Server - Schema Fix Explanation Was this intentionally checked in? — Reply to this email directly, view it on GitHub <#2812 (review)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAB7TGBOEQCEZVBHKVKB7I33XL4KHAVCNFSM6AAAAACIGC5WT2VHI2DSMVQWIX3LMV43YUDVNRWFEZLROVSXG5CSMV3GSZLXHMZTGMRZG43TEOBVGU> . You are receiving this because you were mentioned.Message ID: ***@***.***>

ketema · 2025-10-13T08:16:05Z

Sorry about the .npmrc i will remove it

…

On Mon, Oct 13, 2025 at 04:13 Ketema Harris ***@***.***> wrote: It has mostly happened with Anthropic Claude models and mostly with the Augment.ai agent, but as mentioned I have had it happen with copilot and Claude and once with Gemini in an early 1.5 flash model, although I wasn’t keeping strict count. It just happened enough that I decided to ask why then make the patch. On Sun, Oct 12, 2025 at 21:15 Ola Hungerford ***@***.***> wrote: > ***@***.**** commented on this pull request. > > Agree with Adam that not sure if forcing the values is the right thing to > do here. I did have another question though about what models you're using > when this happens. > ------------------------------ > > In src/sequentialthinking/.npmrc > <#2812 (comment)> > : > > > @@ -0,0 +1,19 @@ > +# Local NPM configuration for Sequential Thinking MCP Server > > Was this intentionally checked in? > ------------------------------ > > In src/sequentialthinking/SCHEMA_FIX_EXPLANATION.md > <#2812 (comment)> > : > > > @@ -0,0 +1,221 @@ > +# Sequential Thinking MCP Server - Schema Fix Explanation > > Was this intentionally checked in? > > — > Reply to this email directly, view it on GitHub > <#2812 (review)>, > or unsubscribe > <https://github.com/notifications/unsubscribe-auth/AAB7TGBOEQCEZVBHKVKB7I33XL4KHAVCNFSM6AAAAACIGC5WT2VHI2DSMVQWIX3LMV43YUDVNRWFEZLROVSXG5CSMV3GSZLXHMZTGMRZG43TEOBVGU> > . > You are receiving this because you were mentioned.Message ID: > ***@***.***> >

Copilot AI review requested due to automatic review settings October 3, 2025 09:52

Copilot AI reviewed Oct 3, 2025

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Merge branch 'modelcontextprotocol:main' into fix/input-sanitization-…

2c85c1d

…sequential-thinking

olaservo added server-sequentialthinking Reference implementation for the Sequential Thinking MCP server - src/sequentialthinking bug Something isn't working labels Oct 8, 2025

ketema added 2 commits October 7, 2025 23:16

ketema force-pushed the fix/input-sanitization-sequential-thinking branch from 28311c0 to ca00790 Compare October 8, 2025 09:05

domdomegg requested changes Oct 10, 2025

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fix(sequential-thinking): Add input sanitization for numeric parameters #2812

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		@@ -0,0 +1,19 @@
		# Local NPM configuration for Sequential Thinking MCP Server

		@@ -0,0 +1,221 @@
		# Sequential Thinking MCP Server - Schema Fix Explanation

fix(sequential-thinking): Add input sanitization for numeric parameters #2812

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fix(sequential-thinking): Add input sanitization for numeric parameters #2812

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ketema commented Oct 3, 2025

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Testing

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ketema commented Oct 3, 2025

✅ Comment 1: Regex inconsistency with zero - FIXED

❌ Comment 2: Negative numbers and decimals - Not applicable

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